Helicopters and other vertical take-off and landing craft (VTOL) are known in the art. The limitations of helicopter designs are also well known. In particular, helicopter rotors generate an enormous amount of aerodynamic drag, which in turn limits the forward speed of a helicopter to much less than that of a conventional fixed-wing aircraft.
Many attempts have been made to overcome this particular deficiency of helicopter design. Probably the most notable attempt has been the limited production of the Bell V-22 Osprey aircraft, which in turn represents the culmination of years of tilt-wing and tilt-rotor research craft. The V-22 Osprey has itself fallen prey to some design limitations. The complex double tilt-rotor mechanism used by the Osprey has failed on occasion resulting in a number of mishaps.
In particular, the V-22 Osprey utilizes two wing-mounted turbine engines, each driving a single large diameter propeller. Each engine drives its corresponding propeller in a direction opposite of the other so as to negate any torque effects. To reduce the likelihood of a crash in the event of a single engine-out situation, the V-22 is also provided with a driveshaft connecting the two engines through a complex gearbox mechanism, such that if one engine fails, the other can drive both propellers. Failure of these complex mechanisms and associated hydraulics hardware has been traced as the cause of some V-22 crashes. A simpler and more robust tilt-rotor design is thus still required in the art.
The V-22 suffers from another design defect as well. Even though the tiltable rotors are mounted on the ends of a high-wing, the craft cannot be landed in horizontal flight mode, except in cases of emergency. If the craft is landed in horizontal flight mode, the blades of the lift/thrust props will strike the ground, destroying the props and possibly causing propeller fragments to injure the crew or bystanders.
Helicopters suffer from yet another design deficiency. In order to provide constant lift throughout the rotation of the main rotor when traveling in a forward direction, a helicopter is generally provided with a swash plate and cyclic control to control the angle of attack of the main rotor through its cycle of rotation. When the rotor is passing in a forward direction in its arc, the angle of attack is reduced. When the rotor is passing in a rearward direction in its arc, the angle of attack is increased. This cyclic control compensates for the difference in lift produced by a helicopter blade when the helicopter is traveling in one direction. However, such swash plate and cyclic controls add complexity to the rotor and hub design, increasing unit cost, maintenance cost and decreasing reliability. In addition, the cyclic control can be difficult to for a pilot to master and requires a different and more complicated skill set which requires extensive pilot training.
Pham, U.S. Pat. No. 6,382,556, issued May 7, 2002, discloses a VTOL airplane with only one tiltable prop-rotor. The Pham design looks like a standard General Aviation (GA) aircraft with a main rotor and tail rotor attached. The tilt mechanism causes the engine and rotor assembly to move from a position aft of the main wing to a position forward of the cockpit. This dramatic tilt moves the Center of Gravity (CG) forward from position 22V to 22H when transitioning from vertical to horizontal flight.
In addition, the amount of space and the weight of the tilting mechanism must be substantial, given the large arc that the engine/rotor assembly travels through. Moreover, as the design is an apparent combination of airplane and helicopter shapes, it presents a fairly long wing and large rotor combination. Such an enormous rotor presents a large amount of drag when in the horizontal flight mode. Pham also suffers from the same deficiency as the V-22 in that it cannot be landed in horizontal flight mode without destroying the main rotor blade, as it will not clear the ground in horizontal flight mode.
Trek Aerospace (www.millenniumjet.com/index.html) discloses a VTOL craft called the SPRINGTAIL(TM) EFV-4. The unit is almost worn by the user in a similar manner to the Bell JetPack of the 1960's. Twin ducted rotors are driven by a rotary engine. A fly-by-wire control system provides directional control to the pilot. It appears that the rotors are tiltable to provide directional thrust. It is not clear how yaw and directional control are obtained. Providing differential thrust could require complex propeller pitch or speed control. The unit does not appear to allow for high-speed horizontal flight.
Thus, what remains a requirement in the art is to produce a VTOL/HTOL/STOL craft having a simple tilt-rotor configuration which can combine the vertical lift-off capabilities of a helicopter with the high speed of a fixed wing aircraft—without increasing complexity and cost over that of a prior art helicopter, and which may take off and land in both vertical and horizontal flight modes.